The ultimate target tissue for mouse mammary tumor virus (MMTV) is the mammary gland, where viral integrations near cellular oncogenes result in mammary tumors. MMTV also uses cells of the immune system in its journey from milk to the mammary gland of offspring that nurse on infected mothers. A better understanding of how MMTV infects these different cells and reaches its target tissue is dependent on identifying the cell surface entry receptor. Moreover, although MMTV only infects rodent tissue culture cells, several groups have recently isolated viral sequences highly related to MMTV from human breast cancer but not normal tissue. Thus, a complete understanding of MMTV's cell-type- and species-tropism is critical. Although we previously cloned a receptor for MMTV (MTVR2), we recently discovered by phenotypic screening of the T31mouse/hamster radiation hybrid cell panel that mouse transferrin receptor 1 (TrfR1) is the major cell entry receptor. The goals of this proposal are to determine how MMTV interacts with this well-characterized receptor in infection, both at the level of the cell and the whole animal. The first of the two proposed aims will take advantage of the TrfR1 crystal structure and the availability of known trafficking mutants to determine species-specific tropism and how MMTV uses this receptor for entry into cells. The second aim will examine in vivo infection in mice that have defects in the TrfR1 gene and in other steps of transferrin/iron metabolism. Together, these two approaches will allow us to better understand how viruses utilize the biology of their host for propagation and how MMTV specifically induces breast cancer in mice.